An investigation of domestic wastewater treatment systems impacts on surface waters and applicability of wastewater fingerprinting compounds

An Investigation of Domestic Wastewater. Treatment System Impacts on Surface Waters. and the Applicability of Wastewater. Fingerprinting Tracers. Laura Brophy . A thesis submitted for the Degree of Doctor of Philosophy to. the University of Dublin, Trinity College. 2017. Department of Civil, Structural and Environmental Engineering. University of Dublin. Trinity College. Dublin. i. Declaration. I declare that thesis has not been submitted as an exercise for a degree at this or any other university. and it is entirely my own work.. I agree to deposit this thesis in the University’s open access institutional repository or allow the. library to do so on my behalf, subject to Irish Copyright Legislation and Trinity College Library. conditions of use and acknowledgement.. Signed:. . ii. Summary. This research estimated the P and N loading to surface water arising from domestic wastewater. treatment systems (DWWTS) and compared a range of wastewater fingerprinting techniques for. identifying sources of surface water contamination. In particular, the aim was to quantify the extent. of pollution from DWWTS as distinct from agricultural sources. . Four small catchments, each containing a high density of DWWTS (15 - 32 DWWTS km -2. ) and. underlain by either poorly permeable subsoil or shallow, impermeable bedrock, were selected for. study. The primary stream in each catchment was monitored upstream and downstream of the main. cluster of DWWTS for Dissolved Reactive Phosphorus (DRP), Total Phosphorus (TP), NO3, NO2, NH4,. TN, Total Coliforms, E.coli and Enterococci. In addition, a number of mid-catchment monitoring. points were also sampled in order to observe changing water quality patterns throughout each. catchment. . A catchment P loading schematic, describing DWWTS-P and -N loading under low flow conditions. was developed. The P loading arising from DWWTS was found to represent between 1.5 % and 9 % of. the annual P loading from the four study catchments. Likewise, between 10 % and 29 % of the annual. N loading was estimated to arise from catchment DWWTS. These results were found to be in close. agreement with those generated by the EPA’s Source Load Apportionment Model (SLAM) software. for the same catchments.. In addition to the estimation of annual loading, general water quality in each catchment was. characterised with respect to P and N fractions and sub-clusters of DWWTS located in close proximity. to the stream were found to result in ‘hotspots’ of severely impacted surface water. . A number of models were examined with the aim of estimating the P contribution of a given DWWTS. as a function of its distance from the stream. Across the four catchments studied, a power inverse. model was found to provide a good description of DWWTS-P loading with distance, describing a. gradual decrease in P loading to the stream with increasing distance. This has potential use in future. mitigation strategies relating to the impacts of DWWTS on surface water. . These catchments have also been used to assess the applicability of a range of fingerprinting. parameters aimed at identifying chronic and incidental contamination arising from domestic. wastewater inputs. This list of chemical and biological domestic wastewater ‘fingerprinting’. compounds comprised ammonium, faecal indicator organisms, human-specific Bacteroidales, faecal. iii. sterol ratios (FSR), fluorescent whitening compounds (FWCs), caffeine, artificial sweeteners and. pharmaceutical compounds. The analytical results of these ‘fingerprinting’ compounds provided a. further level of confidence that there was a measureable presence of domestic effluent in each. catchment. In particular, the artificial sweetener Acesulfame was found to be ubiquitous. downstream of DWWTS. Caffeine was also detected in a large number of samples from each of the. catchments.. Based on observations of the success of the DWWTS tracers examined in this study, a ‘tiered-. approach’ into the investigation of instances of malfunctioning DWWTS is proposed, wherein the. tracers that were studied in this research can be applied in a step-wise manner by environmental. enforcement agencies to isolate the DWWTS(s) causing water quality issues in a given catchment. . This ‘tiered-approach’ makes use of observations made during the examination of the applicability of. these compounds to tracing DWWTS inputs to the surface water environment. . iv. Acknowledgements. I would firstly like to express my sincere gratitude to Prof Laurence Gill and Mr Bruce Misstear, for. providing me with the opportunity to work on this project, and for their patience and invaluable. guidance over the last four years. It was a great honour to work with two such distinguished. professionals in the environmental engineering field. Thanks also to Dr Philip Geary of the University. of Newcastle for his help on the project, and for travelling such great distances to work with us. My. thanks also to the EPA, the main funding organisation for the project, and to the project steering. committee members for providing valuable advice during our numerous committee meetings.. Another mentor of sorts was Dr Donata Dubber, and I am extremely grateful to her for her friendship. and guidance throughout the process, particularly with respect to the long hours of fieldwork and. laboratory work that we shared. Kate Kilroy of NUIG also deserves a mention here, especially for the. times that we spent scrambling through thorny ditches and sliding down river banks to get water. samples! Thanks also to Dr David O’Connell for his work on the compounds of emerging concern. . Setting up my fieldwork stations would have been a great deal more difficult, and a lot less fun,. without the help of Mr Pat Veale, Lab Technician in Trinity College. Thank you for also providing. encouragement and a good laugh when both were needed! I’d also like to thank Dr Kevin Ryan and. all of the Technicians in the Simon Perry building for their generous support – particularly for their. help with the ‘van incident’ that day by the cricket field! . Chris Fennell has been a great support for the past number of years. We began our respective PhD. projects at the same time and I’d like to thank him sincerely for being a good friend and colleague. throughout, and to wish him all the best in his career. The same goes for all of the crew in the office. at the top of the Simon Perry ‘Tower’ – Tracy, Jiayi and especially Elia (from Catalonia)! A huge thanks. also to my colleagues in the Marine Institute, for their help and support in getting the write up. finished.. It’s been a long four years, largely involving me confining myself to the immediate vicinity of my. computer and missing out on events with my very best friends, so I’d like to give them all my thanks. – Tina, Murty, Noelle, Jean, Andrea and Niamh – thanks for sticking with me! I promise to be more. fun (and a lot more reliable) from now on! A special mention also to Erin Cave for our infrequent,. but highly valued, coffee evenings! Also to Steve and the crew of The Engine Room, Portumna – who. unknowingly helped so much in getting me over the final hurdle. . v. I would not have gotten through it all without the support of my family – Mum, Dad, Carol, Ed, Mark,. Stuart, Dee, Louise and the little members of the clan – Sophie, Robert, Rachel, Nathan and (most. recently) Eithne! My special thanks to my parents, Bridie and Pat, for their support.. Finally to Mr Derek Horan, who was always there to support me throughout the last few years and. without whom, it would have been a very different experience. Thanks for all of your help! . Sin e! Go raibh mile maith agaibh!. vi. TABLE OF CONTENTS. 1 Introduction ..................................................................................................................................... 1. 1.1 Background ......................................................................................................................... 1. 1.2 Research aims and objectives ............................................................................................. 8. 1.3 Thesis outline ...................................................................................................................... 9. 2 Literature Review - Sources and Transport of Phosphorus at the Catchment Scale .................... 11. 2.1 Primary sources of phosphorus ........................................................................................ 11. 2.1.1 Domestic wastewater treatment systems (DWWTS)........................................................ 13. 2.1.1.1 DWWTS and catchment water quality – Ireland and the UK ........................................... 14. 2.1.1.2 Estimation and modelling of DWWTS P ........................................................................... 20. 2.1.1.3 Fate and transformation of DWWTS-P in the soil treatment area ................................... 22. 2.1.1.4 Summary of DWWTS impacts on stream water quality ................................................... 25. 2.1.2 Wastewater treatment plants (WWTPs) ........................................................................... 26. 2.1.3 Atmospheric deposition and geogenic sources of P ......................................................... 28. 2.1.4 Agriculture ........................................................................................................................ 30. 2.2 Catchment hydrology and phosphorus pathways - UK and Ireland ................................. 34. 2.3 Conclusion ........................................................................................................................ 36. 3 Literature Review - Tracers of DWWTS Contamination in Surface Waters ................................... 38. 3.1 Faecal indicator organisms – Total Coliforms, E. coli and Enterococci ............................ 39. 3.2 Bacteroidales 16S rRNA detection in surface waters ....................................................... 41. 3.3 Faecal sterol ratios ............................................................................................................ 45. 3.4 Fluorescent Whitening Compounds (FWCs) ..................................................................... 50. 3.5 Caffeine ............................................................................................................................. 53. 3.6 Artificial Sweeteners ......................................................................................................... 56. 3.6.1 Acesulfame (ACE) .............................................................................................................. 56. 3.6.2 Sucralose (SUC) ................................................................................................................. 58. 3.6.3 Saccharin (SAC), Cyclamate (CYC) and Aspartame (ASP) .................................................. 58. 3.7 Pharmaceutical Compounds ............................................................................................. 60. vii. 3.7.1 Carbamazepine .................................................................................................................. 60. 3.7.2 Sulfamethoxazole .............................................................................................................. 62. 3.8 Conclusions........................................................................................................................ 65. 4 Research Methodology ................................................................................................................. 67. 4.1 Study site instrumentation ................................................................................................ 67. 4.1.1 Flow, temperature and electrical conductivity ................................................................. 67. 4.1.2 Meteorological data .......................................................................................................... 69. 4.2 Sampling methodology ..................................................................................................... 72. 4.3 Water quality analysis ....................................................................................................... 75. 4.3.1 Phosphorus ........................................................................................................................ 76. LOD for P fractions ............................................................................................................................ 78. Performance of P standards in QC check .......................................................................................... 78. Repeatability of P fraction analysis ................................................................................................... 78. 4.3.2 Nitrogen............................................................................................................................. 79. Performance of N03 standards in QC check ....................................................................................... 79. Repeatability of N03 fraction analysis ............................................................................................... 80. Nitrite 81. Performance of N02 standards in QC check ....................................................................................... 81. Repeatability of N02 fraction analysis ............................................................................................... 81. Ammonium ........................................................................................................................................ 82. Performance of NH4 standards in QC check ...................................................................................... 82. Repeatability of NH4 fraction analysis ............................................................................................... 83. Total Nitrogen ................................................................................................................................... 84. Performance of N standards in QC check .......................................................................................... 86. Repeatability of N fraction analysis .................................................................................................. 86. 4.3.3 Faecal Indicator Organisms (FIOs) ..................................................................................... 87. 4.4 Bacteroidales ..................................................................................................................... 87. 4.5 Faecal Sterol Ratios ........................................................................................................... 88. 4.5.1 Fluorescent Whitening Compounds (FWCs) ..................................................................... 90. viii. 4.5.2 Caffeine, Artificial Sweeteners and Pharmaceutical Compounds .................................... 91. 4.5.3 General Quality control .................................................................................................... 93. 4.6 Conceptual Model for Estimating DWWTS-P and DWWTS-N Contribution to Stream P and. N 95. 4.6.1 Catchment Flow, Phosphorus and Nitrogen Loading Model ............................................ 96. 4.6.2 Catchment P Loading Model ............................................................................................ 99. 4.6.2.1 Estimation of overall DWWTS-P export .......................................................................... 100. 4.7 Influence of DWWTS Distance to Stream on DWWTS-P Load ........................................ 101. 4.8 Comparison of Estimated DWWTS-P and DWWTS-N with Existing Catchment Modelling. Tools – SLAM and SANICOSE .......................................................................................................... 104. 5 Study Catchment Selection and Description ............................................................................... 106. 5.1 Study catchment selection ............................................................................................. 106. 5.2 Description of Study Catchment 1 (C1) .......................................................................... 110. 5.2.1 Catchment Geology and Soils ......................................................................................... 111. 5.2.2 Land Use ......................................................................................................................... 116. 5.3 Description of Study Catchment 2 (C2) .......................................................................... 119. 5.3.1 Catchment Geology and Soils ......................................................................................... 120. 5.3.2 Land Use ......................................................................................................................... 125. 5.4 Description of Study Catchment 3 (C3) .......................................................................... 127. 5.4.1 Catchment Geology and Soils ......................................................................................... 128. 5.4.2 Land Use ......................................................................................................................... 132. 5.5 Description of Study Catchment 3 (C4) .......................................................................... 134. 5.5.1 Catchment Geology and Soils ......................................................................................... 135. 5.5.2 Land Use ......................................................................................................................... 142. 5.6 Comparison of study sites to a wider Irish context .................................................................. 145. 5.7 Mid-catchment monitoring stations............................................................................... 146. 6 Results - Catchment 1 .................................................................................................................. 151. 6.1. Catchment DWWTS ........................................................................................................ 151. 6.2. Catchment Hydrology ..................................................................................................... 152. ix. 6.2.1. Dominant Flow Pathways in C1 ....................................................................................... 159. 6.2.2. Electrical Conductivity (EC) Record in C1 ........................................................................ 163. 6.2.3. Catchment Hydrology and DWWTS Hydraulic Load ....................................................... 166. 6.3. Catchment Water Quality ............................................................................................... 168. 6.4. Phosphorus ...................................................................................................................... 170. 6.4.1.1. Phosphorus Fractions ...................................................................................................... 172. 6.4.1.2. High Frequency Water Quality Monitoring Events ......................................................... 174. 6.5. Nitrogen........................................................................................................................... 176. 6.6. Microbiology ................................................................................................................... 178. 6.7. Estimation of DWWTS-P in C1 ......................................................................................... 182. 6.7.1. Estimation of overall DWWTS-P export .......................................................................... 187. 6.7.2. Influence of DWWTS distance to stream on DWWTS-P load .......................................... 190. 6.7.3. Estimated N discharged from Catchment DWWTS ......................................................... 194. 6.8. Comparison of Research Data with SLAM outputs ......................................................... 196. 7 Results – Catchment 2 ................................................................................................................. 198. 7.1. DWWTS in the Catchment ............................................................................................... 198. 7.2. Catchment Hydrology ...................................................................................................... 199. 7.2.1. Dominant Flow Pathways in C2 ....................................................................................... 205. 7.2.2. Electrical Conductivity (EC) Record in C2 ........................................................................ 207. 7.2.3. Catchment Hydrology and DWWTS Hydraulic Load ....................................................... 213. 7.3. Catchment Water Quality ............................................................................................... 216. 7.3.1. Phosphorus ...................................................................................................................... 217. 7.3.1.1. Phosphorus Fractions ...................................................................................................... 218. 7.3.1.2. High Frequency Water Quality Monitoring Events ......................................................... 220. 7.3.2. Nitrogen........................................................................................................................... 223. 7.3.3. Microbiology ................................................................................................................... 225. 7.4. Estimation of DWWTS-P and DWWTS-N loading in C2 ................................................... 228. 7.4.1. Estimation of overall DWWTS-P export .......................................................................... 232. 7.4.2. Influence of DWWTS distance to stream on DWWTS-P load .......................................... 234. x. 7.4.3. Estimated N discharged from Catchment DWWTS ........................................................ 237. 7.5. Comparison of Research Data with SLAM outputs......................................................... 241. 8 Results – Catchment 3 ................................................................................................................. 243. 8.1. Catchment DWWTS ........................................................................................................ 243. 8.2. Catchment Hydrology ..................................................................................................... 243. 8.2.1. Dominant Flow Pathways in C3 ...................................................................................... 248. 8.2.2. Electrical Conductivity (EC) Record in C3........................................................................ 249. 8.2.3. Catchment Hydrology and DWWTS Hydraulic Load ....................................................... 254. 8.3 Catchment Water Quality ........................................................................................................ 255. 8.3.1 Phosphorus .......................................................................................................................... 255. 8.3.1.1 Phosphorus Fractions .......................................................................................................... 257. 8.3.2 Nitrogen ............................................................................................................................... 258. 8.3.3 Microbiology ........................................................................................................................ 260. 8.4 Estimated DWWTS-P and DWWTS-N loading in C3 ................................................................ 261. 8.4.1 Estimation of overall DWWTS-P export ............................................................................... 265. 8.4.2 Influence of DWWTS distance to stream on DWWTS-P load .............................................. 268. 8.4.3 Estimated N discharged from Catchment DWWTS ............................................................. 272. 8.5 Comparison of Research Data with SLAM outputs ................................................................. 275. 9 Results - Catchment 4 .................................................................................................................. 277. 9.1 DWWTS in the Catchment ....................................................................................................... 277. 9.2 Catchment Hydrology .............................................................................................................. 278. 9.2.1 Dominant Flow pathways in C4 ........................................................................................... 285. 9.2.2 Electrical Conductivity (EC) Record in C4 ............................................................................ 286. 9.2.3 Catchment Hydrology and DWWTS Hydraulic Load ............................................................ 289. 9.3 Catchment Water Quality ........................................................................................................ 291. 9.3.1 Phosphorus .......................................................................................................................... 291. 9.3.1.1 Phosphorus Fractions .......................................................................................................... 293. 9.3.1.2 High Frequency Water Quality Monitoring Events ............................................................. 297. 9.3.2 Nitrogen ............................................................................................................................... 298. 9.3.3 Microbiology ........................................................................................................................ 300. 9.4 Estimated DWWTS-P and DWWTS-N loading in C4 ................................................................ 301. 9.4.1 Estimation of overall DWWTS-P export ............................................................................... 303. 9.4.2 Influence of DWWTS distance to stream on DWWTS-P load .............................................. 305. 9.4.3 Estimated N discharged from Catchment DWWTS ............................................................. 309. xi. 9.5 Comparison of Research Data with SLAM outputs ................................................................. 312. 10 Results - Tracers of Domestic Wastewater in the Surface Water Environment ..................... 314. 10.1 Ammonium ...................................................................................................................... 314. 10.2 Faecal Indicator Organisms ............................................................................................. 324. 10.3 Bacteroidales ................................................................................................................... 329. 10.4 Faecal Sterol Ratios ......................................................................................................... 344. 10.5 Fluorescent Whitening Compounds ................................................................................ 360. 10.6 Compounds of Emerging Concern (CECs) ....................................................................... 370. 10.6.1 Caffeine ........................................................................................................................... 371. 10.6.2 Artificial Sweeteners ....................................................................................................... 377. 10.6.3 Pharmaceutical Compounds ........................................................................................... 386. 11 Discussion ................................................................................................................................ 394. 11.1 Evidence of DWWTS influence on water quality ............................................................. 394. 11.1.1 Catchment Hydrology and EC .......................................................................................... 394. 11.1.2 General Catchment Water Quality .................................................................................. 396. 11.1.2.1 Phosphorus ...................................................................................................................... 396. 11.1.2.2 Nitrogen........................................................................................................................... 398. 11.1.2.3 Microbial Data ................................................................................................................. 399. 11.2 Estimation of DWWTS P and DWWTS N ......................................................................... 404. 11.3 Attenuation of DWWTS P as a Function of Distance from Stream ................................. 409. 11.4 Comparison of Research Data with SLAM data ............................................................... 414. 11.5 DWWTS Tracer Compounds – Summary and Discussion ................................................ 419. 11.5.1 Ammonium ...................................................................................................................... 419. 11.5.2 Faecal Indicator Organisms ............................................................................................. 419. 11.5.3 Bacteroidales ................................................................................................................... 420. 11.5.4 Faecal Sterol Ratios ......................................................................................................... 423. 11.5.5 Fluorescent Whitening Compounds ................................................................................ 425. 11.5.6 Caffeine ........................................................................................................................... 426. 11.5.7 Artificial Sweeteners ....................................................................................................... 428. xii. 11.5.7.1 Acesulfame-K (ACE) ........................................................................................................ 428. 11.5.7.2 Sucralose (SUC) ............................................................................................................... 429. 11.5.7.3 Saccharin (SAC) ............................................................................................................... 429. 11.5.7.4 Cyclamate (CYC) .............................................................................................................. 429. 11.5.7.5 Aspartame (ASP) ............................................................................................................. 429. 11.5.8 Pharmaceutical Compounds ........................................................................................... 430. 11.5.8.1 Carbamazepine (CBZ) ..................................................................................................... 430. 11.5.8.2 Sulfamethoxazole (SMX) ................................................................................................. 431. 11.6 Discussion of Tracer Data ............................................................................................... 431. 11.7 Impacts of DWWTS on Surface Water – Proposed Investigative Strategy ..................... 434. 12 Conclusions and Recommendations........................................................................................ 438. References ........................................................................................................................................... 442. Appendix A - Complete study catchment scoring system ................................................................... 460. Appendix B - Results of study catchment shortlisting process............................................................ 462. Appendix C – 7-hourly sample collection schedule ............................................................................. 463. Appendix D High-level geographic location of the four study catchments included in this study* ... 464. xiii. List of Abbreviations. CoP – Code of Practice (EPA, 2009). DRP – Dissolved Reactive Phosphorus. DuRP – Dissolved Unreactive Phosphorus. E.C. – Electrical Conductivity. EQS – Environmental Quality Standard. LIPTG – Likelihood of Inadequate Percolation to Groundwater. nd – non-detect. nt – not tested. P.E. – Population Equivalent. PP – Particulate Phosphorus. S.T.A. – Soil Treatment Area. TDP – Total Dissolved Phosphorus. TOC – Total Organic Carbon. TP – Total Phosphorus. Very High LIPTG – Very High Likelihood of Inadequate Percolation to Groundwater. WFD – Water Framework Directive. SLAM – Source Load Apportionment Model. 1. 1 Introduction. 1.1 Background Despite advances in wastewater treatment research in recent years (Gill et al., 2009; Mahjouri et al.,. 2017) and research into alternative modes of disposal in rural locations (e.g. Dubber and Gill, 2014), a. large proportion of the world’s population continues to rely on basic forms of decentralised Domestic. Wastewater Treatment Systems (DWWTS) for disposal of domestic wastewater (see Table 1.1). . For example, D’Amato et al. (2008) report that 23% of the 115 million households in the United. States rely on DWWTS, equating to 26,450,000 active systems. In Australia, 2 million people (13% of. the population) rely on DWWTS (Thomas et al., 1997). In France, 18% of the population are not. connected to a collective sanitation system. This is equivalent to approximately 5 million households,. or 12 million people (Demouliere et al., 2012). Conversely, in a more traditionally urbanised country,. in the United Kingdom, it is estimated that only 4% of the population utilise DWWTS for domestic. wastewater disposal (DEFRA, 2012), which equates to approximately 2,605,600 people (Table 1.1).. In Central and Eastern Europe (CEEP), 42 million people (30% of the population) are estimated to live. in small rural settlements (< 2,000 population), of which just 9% are connected to a centralised. WWTP (Istenic et al., 2015). Dependence upon DWWTS is comparatively higher in less-developed. geographical regions. For example, the percentage of urban households utilising septic tanks is 77%. in Vietnam, 90% in Sri Lanka and 62% in Indonesia (Shi and Koné, 2010). . Table 1.1 Illustration of the reliance on DWWTS across four developed countries, as a percentage of the overall. population.. Country Estimated Population relying on DWWTS. % of Population relying on DWWTS. Reference. USA 68,770,000 21 D’Amato et al. (2008) . Australia* 2,000,000* 13 Thomas et al., 1997 France 12,000,000 18 Demoulier et al., 2012 UK 2,665,600 4 DEFRA, 2012 Ireland 1,451,600 31 CSO, 2016 *This figure is likely to be closer to 3,000,000 when based on present-day population statistics. DWWTS constitute a significant part of the water infrastructure of Ireland. The 2016 Irish population. census (CSO, 2016) indicated that almost 500,000 private households utilise a DWWTS for disposal of. domestic wastewater, which equates to 1.45 million people (31 % of the total population) – see. Figure 1.1. This indicates a higher dependence on DWWTS by the Irish population when compared. with other economically-developed countries, but reflects the fact that a high portion of the Irish. population still live in rural areas, rather than indicating that dense population centres are not. serviced by a centralised wastewater collection system. In fact, Ireland’s dependence upon DWWTS. 2. may even be slightly underestimated, as more than 77,000 households surveyed in 2016 did not. define their mode of wastewater disposal. . Whilst recent research has highlighted potential alternatives to the current DWWTS infrastructural. model (Dubber et al., 2014) it is unlikely, excepting major changes in legislation, that the level of. dependency on DWWTS in Ireland and worldwide will change appreciably in the near future. . Figure 1.1 Domestic wastewater disposal facilities of the Irish population (CSO, 2016). Graph A illustrates the. number of households utilizing each treatment type. Graph B illustrates the number of individuals (and percentage of. the overall population) utilizing each treatment type.. *‘Other DWWTS’ includes secondary treatment units and packaged units.. Domestic wastewater contains a broad range of potentially harmful contaminants at high. concentrations. Of particular relevance are phosphorus (P), nitrogen (N), pathogenic micro-. organisms and a wide range of compounds of emerging concern (CECs), the environmental effects of. which are not yet fully studied. Phosphorus (P), as the limiting growth nutrient in freshwater bodies. (Dillon and Rigler, 1974), has the potential to cause significant negative aesthetic and ecological. impacts on water quality if released to the environment.. Based on mathematical modelling and small-scale surveys, Gilmour et al. (2008) estimated that the P. load in domestic wastewater arising from laundry and dishwasher detergents plus human wastes. resulted in a total daily loading of 1.89 g-P person -1. , equivalent to 0.69 kg-P person -1. yr -1. . Similarly,. direct measurement of DWWTS influent in IreIand, reported in Gill et al. (2009), indicated that the. average ortho-P loading to six DWWTS was 1.45 g ortho-P person -1. day -1. (equivalent to 0.53 kg ortho-. P person -1. yr -1. ). . The design capacity of DWWTS in Ireland is based on an estimated wastewater discharge volume of. 150 L person -1. day -1. (EPA, 2009). However, various studies involving direct measurement of DWWTS. Public Sewerage Scheme. (1,118,401). Individual Septic Tank (438,319). Other DWWTS* (60,857). System Undefined. (77,822). No Sewerage Facility (2,266). Public Sewerage Scheme. (2,996,956) 64%. Individual Septic Tank (1,249,025). 27%. Other DWWTS* (202,587). 4%. System Undefined (213,431). 4.6%. No Sewerage Facility (4,377). 0.1%. A. Households B. Population. 3. influent have found that the average discharge figure is considerably lower, at 96.5 L person -1. day -1. . (Gill et al 2009) and 101 L person -1. day -1. (Dubber and Gill, 2014). . Using the more recent figure of 101 L person -1. day -1. , it is estimated that approximately 1.5 x 10 8 L of. domestic effluent is discharged to DWWTS every day in Ireland. Whilst the observed concentration of. contaminants in domestic wastewater varies considerably per capita (e.g. Gill et al., 2009), it is. possible, using the daily wastewater volume and an average P loading per person, to approximate. the loading of P in wastewater generated by the population of Ireland on a daily basis (Figure 1.2). . Figure 1.2 Daily volume of wastewater (A) and mass of ortho-P (B) discharged to Wastewater Treatment Plants. (WWTPs), DWWTS and to undefined receptors based on average ortho-P loading of 1.45 g P person -1. day -1. which. was measured across six DWWTS (Gill et al., 2009), and population data from CSO 2016 population data.. Based on an average ortho-P loading of 1.45 g-P person -1. day -1. (Gill et al., 2009) it is therefore. estimated that 2,105 kg ortho-P is discharged to DWWTS each day in Ireland. This P loading to. DWWTS is highly significant, given that it is almost half of the loading received by WWTPs, which. must adhere to strict limits on the quality of effluent discharged to the receiving environment. following treatment. Meanwhile, whilst performance standards exist for commercial DWWTS with. respect to BOD, SS, NH4, TN and TP, there is no easy way to measure the performance of the system. once it is installed underground. Monitoring of DWWTS has only recently begun in earnest as part of. the National Inspection Programme, but only at a relatively small scale of 1,000 inspections per year. (EPA, 2015a). . Whilst commercial DWWTS are built to high performance standards – generally in line with the I.S.. EN 12566-3:2005 standard (EPA, 2009) - the removal of contaminants such as P from DWWTS. effluent is generally not targeted as part of the treatment process and also greatly depends upon the. correct construction, location and maintenance of the system.. DWWTS, in their most basic form (Figure 1.3), consist of a septic tank connected to a network of. discharge pipes which channel wastewater from the household to a primary settlement tank, where. solids are removed from the influent and anaerobic disgestion of some contaminants occurs. The. supernatant discharges from the tank to be distributed across a network of underground percolation. 4.7E+08. 3.0E+08. 1.5E+08. 2.2E+07 4.4E+05. 0.0E+00. 5.0E+07. 1.0E+08. 1.5E+08. 2.0E+08. 2.5E+08. 3.0E+08. 3.5E+08. 4.0E+08. 4.5E+08. 5.0E+08. Total Population. WWTP DWWTS Undefined Treatment. System. No Treatment. System. V o. lu m. e o. f W. a st. e w. a te. r G. e n. e ra. te d. [ L. d a. y -1. ]. 6,766. 4,346. 2,105. 309 6. 0. 1,000. 2,000. 3,000. 4,000. 5,000. 6,000. 7,000. 8,000. Total Population. WWTP DWWTS Undefined Treatment. System. No Treatment. System. o rt. h o. -P l. o a. d in. g [. k g. d a. y -1. ]. A B. 4. pipes which spread the effluent over the soil treatment area (STA) / percolation area. The size of the. STA is dependent on the household occupancy rate, and is built to standards specified in the EPA. Code of Practice (EPA, 2009). . In order for the DWWTS to function correctly, an adequate depth of unsaturated subsoil beneath the. base of the percolation trenches is vital, to allow for the vertical movement of the effluent through. the soil matrix, wherein attenuation of the constituents of the domestic wastewater loading – such. as Phosphorus and micobial organisms – occurs. . In line with the aforementioned Code of Practice (EPA, 2009) (CoP), the depth of unsaturated subsoil. and the soil drainage properties of a given location are examined in a site suitability assessment,. prior to permission being granted for DWWTS construction. The CoP stipulates that at least 1.2 m of. unsaturated, permeable subsoil beneath the base of the percolation trench is required for septic tank. effluent disharge. Soil drainage conditions are assessed by means of a falling head test (T test), which. measures the amount of time taken for a given depth of water to drop by 25 mm. A T test result. between 3 and 50 indicates that the site is suitable for a conventional DWWTS. . Figure 1.3 Schematic of a DWWTS under ideal operating conditions, illustrating the vertical percolation of effluent. through the subsoil of the STA (adapted from EPA, 2009). T test results greater than 90 indicate that the site is poorly drained, and therefore unsuitable for a. conventional DWWTS. In practice, as evidenced by an examination of planning applications in regions. where drainage conditions are poor, planning permission is often granted for a DWWTS in spite of a. failed T test, on foot of the submission of design proposals for an altered treatment system which. can provide adequate attenuation of effluent despite poorly-draining conditions (e.g. raised. percolation mounds). . 5. Poorly drained sites (usually located where soils and subsoils have a high clay content – Meehan and. Lee, 2012) are unsuitable for the location of DWWTS, as they do not have the capacity to allow. adequate percolation of effluent. Where systems have been constructed in poorly drained areas,. direct pipeline connections between the septic tank and a nearby stream or drainage ditch are. common, in order to avoid hydraulic failure of the system (e.g. McCarthy et al., 2012). Another. feature of DWWTS in poorly drained catchments is the ponding of untreated effluent at, or close to,. the ground surface and its lateral migration towards the nearest surface water receptor. The initial. results of 1,000 inspections carried out under the National Inspection Plan (EPA, 2015b), which aims. to inspect and, if necessary, effect repairs to failing DWWTS, indicated that 489 systems failed the. inspection. Of these, 170 involved illegal direct connections to surface waters, and 115 involved. incidences of surface ponding of effluent, due to inadequate percolation conditions. . Poorly drained catchments are very common in Ireland. A GIS map of the Likelihood of Inadequate. Percolation to Groundwater (LIPTG) in Ireland, illustrated in Figure 1.4, has been developed by the. EPA (EPA, 2013) by combining digitally-mapped shapefiles of groundwater vulnerability, subsoil. permeability (digital shapefile from the groundwater section of the Geological Survey of Ireland. (GSI)) aquifer productivity and soil classification. The classification system for assessing LIPTG ranges. from low LIPTG where conditions are ideal for conventional DWWTS, to Very High LIPTG, where. conditions are unsuitable for adequate DWWTS operation, and is fully described in Table 1A,. Appendix 1 of the EPA-published document “A Risk-Based Methodology to Assist in the Regulation of. Domestic Wastewater Treatment Systems” (EPA, 2013). . Of the 500,000 DWWTS in operation in Ireland (CSO, 2016), 140,410 (28%) are located on sites with. Very High LIPTG, where scenarios such as that illustrated in Figure 1.5 are likely – i.e. inadequate. drainage conditions result in the lateral migration of poorly treated effluent from the STA to the. nearest surface water receptor (e.g. a drainage ditch or a stream) via overland flow or shallow. subsurface pathways. Hence, the risk of pollution of surface waters in these areas of Very High LIPTG. arising from DWWTS is significant. . Under Ireland’s Water Framework Directive (WFD) commitments, ‘Good’ ecological status of all. water bodies must be established and maintained. Achieving this goal requires an understanding of. pollutant contributions from all catchment sources. P concentration of surface waters is one of the. main parameters used to assess water quality. The EQS for P in surface waters in Ireland is 0.035 mg-. P L -1. . The European Communities Environmental Objectives (Surface Waters) Regulations of 2009 (S.I.. 272 of 2009) stipulate that the ecological health of a river water body is assessed with respect to. molybdate reactive phosphorus (MRP) concentration. In order to be designated as a High Status. river, MRP must be ≤0.025 mg-P L -1. (annual mean) or ≤0.045 mg-P L -1. (annual 95%-ile). In order to be. 6. designated as a Good Status river, MRP must be ≤0.035 mg-P L -1. (annual mean) or ≤0.075 mg-P L -1. (annual 95%ile). Thus, quantifying the P loading from all potential catchment sources and. understanding how each source can be mitigated is essential. The daily ortho-P loading of the. 140,410 DWWTS located in areas of Very High LIPTG is estimated at 581 kg ortho-P day -1. , based on. an average occupancy in private households of 2.7 people (CSO, 2016) and a P loading of 1.45 g-P. person -1. day -1. (Gill et al., 2009).. Figure 1.4 Likelihood of Inadequate Percolation to Groundwater (LIPTG) across Ireland. Large areas of Counties. Wexford, Monaghan, Cavan, Longford and Leitrim are characterized as Very High LIPTG (represented by in dark. green), where a conventional DWWTS is unlikely to provide adequate treatment of domestic wastewater (adapted. from EPA, 2013).. The fate of this P load, once it is discharged to DWWTS, represents a poorly-understood element of P. budgets at the catchment scale. Therefore, this research focusses on examining water quality in. small, poorly drained rural catchments which are densely populated with DWWTS, with a view to. determining to what extent DWWTS impact water quality, particularly with respect to P, and to. identify analytical tools for monitoring potential DWWTS impacts.. The source-pathway-receptor model (see Figure 1.5) describing the transfer of pollutants from. source to receptor underpins this research study. The study areas involved were small, rural. catchments with no major industry aside from agriculture and some small scale forestry. Given that. 7. the land quality was generally poor, the dominant agricultural practice was pasture for the dairy and. beef industries, and grazing for horses. In these types of ‘off-the-grid’ catchments (i.e. no mains. drinking water supply or sewage collection system), the major sources of pollution to surface and. groundwater bodies therefore arise from agricultural practices, but also from DWWTS. The main. pollutants of concern in such catchments – though by no means the only pollutants at play – were N. and P, given the potential of these two nutrients to detrimentally impact coastal waters and. freshwaters, respectively. . In general, all agricultural practices have the potential to result in pollution of waterways. In arable. catchments, ploughing of land in preparation for planting crops can result in washout of sediment. with high P content, and washout of fertilizers during the growing season. In pasture catchments,. there is also the potential for washout of P-heavy sediment during rainfall, but in addition, there are. risks to surface water bodies from animals getting direct access to rivers and streams for watering,. and from farmyards acting as point sources of pollution due to animal slurry. Grazing pastures are. also subject to fertilizer application – often in the form of animal slurry which has been stored in. slatted sheds during the winter months and applied to the land at the start of spring and through to. summer. Improvements to poor-quality agricultural land, such tile and open field drains in poorly-. drained catchments, also contribute to water pollution due to the fact that both represent rapid. transport pathways from the source, e.g. a ploughed field in heavy rain draining into a man-made. field drain, to the receptor, e.g. the surface water body into which the drain feeds.. A major, long-term project on the identification of the primary contaminant transfer pathways from. source to receptor in four contrasting Irish river catchments (Archbold et al., 2016) observed that. four broadly-defined pathways can be identified at the catchment scale: overland flow, interflow,. shallow groundwater flow, deep groundwater flow. Two further pathways, of greater importance in. catchments underlain by poorly productive aquifers – the transitional zone (TZ) and artificial drainage. networks – were also identified as important pathways with respect to nutrient transfer, with the TZ. functioning sometimes as part of interflow and sometimes as shallow groundwater flow. With. respect to P transport from source to receptor, Archbold et al. (2016) report that overland flow. transported the bulk of catchment P load, in the particulate and soluble fractions. In terms of this. study and the catchments involved, two other pathways can be identified – direct discharge of. domestic wastewater into the nearest surface water body – usually via a pipeline into a drain or. stream. In addition, preferential flow paths are likely to be an issue with respect to the transfer of. DWWTS effluent in poorly-draining subsoil.. The primary receptors at risk are surface water bodies – in this case small catchment streams and the. larger tributaries and subsequent standing water bodies which receive input from these small. 8. headwaters – and groundwater. Whilst the latter is not the not the primary receptor of concern in. this study, it represents an important facet of the source-pathway-receptor model. The former. represents the type of receptor that is examined in this study with respect to DWWTS impacts.. Figure 1.5 Representation of a DWWTS in unsuitable percolation conditions. Poorly drained subsoil results in the. lateral migration of DWWTS effluent from the STA to a nearby surface water receptor via the shallow subsurface. pathway or via overland flow. Other catchment sources of pollution and transfer pathways are also illustrated.. 1.2 Research aims and objectives A number of questions arise as a result of the widespread distribution of these point contamination. sources. Firstly, does the presence of DWWTS alter catchment surface water quality? If so, how much. of the catchment P load is potentially associated with DWWTS? Which markers of domestic. wastewater are most suitable for the investigation of DWWTS pollution of surface waters? Is distance. of DWWTS from the stream or DWWTS density the more relevant factor with respect to the level of. impact of DWWTS on surface water quality.. The primary aim of this research project was to study the potential impacts of DWWTS on surface. water quality with respect to estimating the contribution of DWWTS to in-stream P and N loading. A. second aim of the research is to identify a suite of suitable chemical, biochemical and microbiological. tracers to help with the detection of domestic wastewater inputs to surface water bodies. . Towards this end, four small, densely-populated head water catchments were selected for intensive. study. The objectives of the research were then to:.  Characterise the hydrology and surface water quality of these four catchments with high. densities of DWWTS located on low permeability soils..  Assess P and N loading from DWWTS to the surface water receptor using catchment. hydrology, in-stream nutrient concentrations and digital mapping.. Topsoil. Subsoil (high clay content) Water Table. Soil Treatment Area (STA). Primary. Tank. Domestic Wastewater Treatment System (DWWTS). Surface Water. Receptor. Pathway 1. Pathway 3. Pathway 1 – Overland flow Pathway 4 – Artificial Drains Pathway 2 – DWWTS Direct Discharge Pipe Pathway 5 – Transitional Zone Pathway 3 – Interflow and preferential flow paths Pathway 6 – Shallow Groundwater Flow. Pathway 4. Pathway 6. Pathway 5. Agricultural Pollution Sources. Bedrock. Transitional Zone. Pathway 2. 9.  Compare P and N loading on surface water from DWWTS across the different catchments..  Evaluate a suite of DWWTS tracer compounds for future application by environmental. enforcement agencies in assessment of DWWTS surface water impacts..  Develop a tiered approach to investigating inputs of domestic wastewater to surface waters. using the selected tracer compounds.. These objectives have been achieved through the following activities:.  Screening of several catchments using characteristics such as subsoil permeability, DWWTS. location and density, geology and aquifer type, land-use, which are likely to promote a. heightened risk of impact on river water quality from DWWTS..  Selection of four small study catchments which featured these conditions, utilising digital. mapping software, and existing digital data files..  Instrumentation of each catchment such that catchment hydrology and water quality. (particularly P, N and microbiology) were monitored upstream and downstream of DWWTS. throughout one year, across a range of hydrological conditions..  Calculation of P loading of DWWTS to surface waters using field observations..  Review of a suite of tracers which are linked with domestic wastewater to evaluate their. strengths, weaknesses and applicability to tracing domestic wastewater inputs to surface. water quantitatively and qualitatively. .  Analysis of the suite of tracers in conjunction with the overall water monitoring programme. in the study catchments, at suitable timescales. Analysis of tracer compounds was carried out. by members of the overall project research team as described in Chapter 4.. 1.3 Thesis outline The scope of this thesis is structured as follows:.  Review of available literature regarding sources of phosphorus at the catchment scale. (Chapter 2)..  Review of hydrological pathways and the application of hydrological models to catchment. (Chapter 2)..  Review of the current literature relating to the use of chemical and molecular markers of. domestic effluent in the surface water environment (Chapter 3)..  Outline of research methodology regarding selection of suitable study catchments,. instrumentation and monitoring schedule and laboratory analysis techniques (Chapter 4).. 10.  Description of the study catchment selection process and a description of each catchment. with respect to DWWTS density, hydrogeology and land use characteristics (Chapter 5)..  Description of the four study catchments with respect to land use, hydrology and DWWTS. density and general water quality (Chapters 6 – 9)..  Estimation of the contribution of DWWTS to catchment P and N load and comparison with. SLAM results (Chapters 6 - 9)..  Assessment of the suite of DWWTS tracer compounds in each study catchment and their. potential applicability to a tiered approach to DWWTS pollution investigation (Chapter 10)..  Discussion (Chapter 11) .  Conclusions and recommendations (Chapter 12).. 11. 2 Literature Review - Sources and Transport of. Phosphorus at the Catchment Scale This review of phosphorus (P) losses to surface waters aims to provide a summary of current. scientific knowledge regarding sources of P at the catchment scale, particularly with respect to. DWWTS-associated P. In addition, previous measures taken to quantify the P loading from DWWTS. are examined with a view to providing a synthesis of the challenges and opportunities for research in. this area. Current knowledge regarding the hydraulic pathways between source (i.e. DWWTS) and. catchment receptors (surface water), and the fate of DWWTS-P in the soil treatment area (STA) are. also reviewed. . 2.1 Primary sources of phosphorus Phosphorus is a naturally-occurring compound in surface water, necessary for the growth and. metabolism of aquatic organisms. However, a surplus of bioavailable P in surface water bodies can. have a number of detrimental effects. Research has linked anthropogenic sources of P with excessive. growth of cyanophytes, macrophytes and dinoflagellates in surface water, leading to the ‘cultural. eutrophication’ and degradation of these water systems. Therefore, control and reduction of in-. stream P concentrations is an important facet of successful water pollution mitigation strategies. (Stamm et al. 2013) and for achieving the objectives of the WFD (Directive 2000/60/EC). . Increasing human population has given rise to greater volumes of treated wastewater discharging. from wastewater treatment plants (WWTPs) and to increased density of DWWTS in rural areas,. which also potentially act as point sources of P, albeit on a different scale to WWTPs. Concurrently,. larger population sizes also require greater food production with corresponding increases in the land-. area under agricultural use and greater agricultural productivity, the latter largely achieved through. the application of artificial and organic fertilizers. This also results in the necessity for the disposal of. animal wastes produced by a growing density of livestock – often via landspreading. . There are a number of sources of P loss at the catchment scale. Current scientific understanding of. these sources is explored in subsequent sections of this chapter.. In this research study, the concentrations of five P fractions were determined; dissolved reactive P. (DRP); total dissolved P (TDP), dissolved unreactive P (DuRP), particulate P (PP) and total P (TP).. DRP is the P fraction in a sample of water filtered through a 0.45 m membrane and directly. measured spectrophotometrically. This fraction is highly bioavailable to freshwater organisms and is. therefore of concern if present at high concentrations. . 12. TDP is the phosphorus fraction in a water sample that has been filtered through a 0.45 M. membrane and subjected to oxidative digestion. Following digestion, the P concentration is. determined spectrophotometrically. This fraction is quantified with the goal of enabling the. concentration of the Dissolved Unreactive P (DuRP) and Particulate P (PP) fractions to be. determined. . The TDP less the DRP fraction is referred to as the Dissolved Unreactive P (DuRP) fraction – i.e. the. portion of the P content of a given water sample that is likely to be colloidally-bound and. conditionally bioavailable to freshwater organisms, depending on physico-chemical conditions of the. water body in question. . DRP, TDP and DuRP in a water sample are likely to become slightly reduced in concentration in the. interval between sampling and analysis, as the dissolved fractions tend to become bound to the sides. of the sample container and to particulate matter within the sample. As such it is necessary to. analyse this fraction immediately upon sampling – or as close as possible to it – in order to accurately. determine each fraction. . Total P (TP) refers to all P present in a given water sample, including P bound to sediment and. colloidal material. TP is generally relatively stable, although there is a potential for a loss of P due to. adsorption to the sides of the sample container. . PP is determined as the TP less TDP. It represents the P in a given sample that is bound to particulate. matter such as sediment or small organic particles. The PP fraction can increase in a standing sample. if there is prolonged period of time between sample collection and analysis, as adsorption of the. dissolved P fractions to suspended matter will increase the concentration of this fraction.. Many environmental enforcement agencies monitor waters for the DRP and TP fractions. The latter. tends to overestimate biologically available P (BAP) fraction of TP, i.e. the portion of TP that is of. most concern with respect to water quality, whereas the former tends to underestimate it (e.g.. Reynolds & Davies 2001). . The distribution of P fractions arising from DWWTS effluent inputs to surface water is of interest to. this study with respect to potentially characterizing DWWTS influence due to the observed. dominance of one fraction over another. However, the pathway between source and receptor is. likely to influence dominant fraction; direct discharges are likely to be dominated by PP, whereas. diffuse discharges – e.g. those passing through the shallow subsurface and undergoing a degree of. attenuation are likely to be dominated by the dissolved fractions. Subsurface pathways including. interflow, artificial drainage networks (tile drainage) and the Transitional Zone (TZ) pathway. (Archibold et al., 2010), are important hydological pathways for flow contribution and nutrient. 13. delivery to streams (Figure 1.5). Where these pathways characterise the hydrological connectivity of. a given catchment, the DRP fraction tends to dominate and may be detected at high levels during low. flow periods due to the lack of attenuation from baseflow (Archibold et al., 2010). However, direct. discharges to stream and to land drains (i.e. ditches dug along the periphery of fields) are likely to be. dominated by the PP draction, given that the only barrier to all P generated in a given household. entering the stream is a loss of the solids portion due to settlement in the primary tank. . In line with the aims of studying characteristics of water quality in catchments with high DWWTS. density, it is important to study the potential effects of inputs of domestic wastewater to the. catchment stream upon these P fractions in the catchment stream. In the poorly-drained catchments. involved in this study it will be of interest to study whether or not there are distinctive observable. patterns with respect the P frations detected. . 2.1.1 Domestic wastewater treatment systems (DWWTS). The discharge of P in its various fractions from centralised waste water treatment plants (WWTPs) to. streams and its contribution to annual P loading is relatively easy to quantify, as both effluent. discharge volume and the hydrological pathway can be accurately defined. This is not the case with. DWWTS. The amount of P exported by these small point sources is difficult to quantify due to a wide. range of variables such as the quality and quantity of wastewater generated, the construction and. maintenance of the DWWTS, the characteristics of the underlying soil treatment area (STA) with. respect to drainage and soil mineralogy, and finally the nature of the hydraulic pathways that exist. between the STA and the receiving groundwater or surface water body. The bulk of the DWWTS. process occurs beneath the ground surface and therefore, the effectiveness of these systems is not. readily measurable.. The presence of a suitable depth of unsaturated subsoil matrix is a crucial element to the treatment. process. As previously mentioned in Chapter 1, approximately 148,500 DWWTS in Ireland are. constructed upon soil that is likely to be saturated for most of the year, making them unsuitable. locations for DWWTS. Figure 2.1 below illustrates a common situation in rural Ireland, highlighting. the risk to surface water posed by the construction of a DWWTS on poorly-drained, saturated. subsoil. . 14. Figure 2.1 Source-Pathway-Receptor model for the transfer of DWWTS contaminants to water receptors on via. percolation to groundwater via the near-surface pathway – the latter due to the presence of thick, saturated and. impermeable subsoil (EPA, 2013).. Aside from the potential threat to surface water quality in terms of P loading, DWWTS also represent. risks to human health and in-stream biota, particularly with respect to the enrichment of surface. waters with pharmaceutical compounds and their potential impacts on ecosystem structure and. stability. For example, in a study by Lambert et al. (2016) higher levels of Endocrine Disrupting. Chemical (EDC) Estradiol (an estrogen hormone) than normal were observed in male frogs in. catchments serviced by DWWTS compared to catchments serviced by WWTPs. Rosi-Marshall et al.. (2013) observed suppressed growth of in-stream bio-films (algae, fungi, bacteria and organic matter). where pharmaceutical compounds were detected at high concentrations. In specific terms, caffeine. suppressed algal growth by 22 % and biofilm respiration by 53 % (Rosi-Marshall et al., 2013). . DWWTS are also thought to be a contributing factor in the decline in riverine salmonid stocks, due to. their contribution to the accumulation of sediment organic matter in spawning grounds which. negatively impacts their viability (Collins et al., 2013).. 2.1.1.1 DWWTS and catchment water quality – Ireland and the UK While the potential impact of DWWTS as point sources of P – particularly with respect to elevating. in-stream DRP - was recognized in early research (e.g. Muscutt and Withers, 1996), these small point. sources were not thought to represent a significant threat to surface water quality, when compared. with discharges from WWTPs. However, most rural parts of Ireland are not serviced by a central. WWTP and as such, there is potential for a measurable impact from DWWTS on surface waters (e.g.. Siegrist et al., 2005).. 15. A number of previous studies on the interactions of DWWTS and catchment water quality are. particularly notable, with respect to highlighting the current knowledge base of DWWTS impacts on. surface water. Arnscheidt et al. (2007) examined water quality in three grassland sub catchments (3 –. 5 km 2 ) of the River Blackwater in Northern Ireland, with the aim of categorising the sources of low. flow P, particularly with respect to the potential for P transfers from DWWTS to surface waters. Poor. soil drainage meant that the catchments featured dense networks of artificial drains, which provided. rapid hydraulic (and contaminant) pathways from the land surface to the catchment stream.. Catchment DWWTS densities were 9.6 km -2. , 13.8 km -2. and 3 km -2. for Armagh (3 km 2 ), Monaghan (5. km 2 ) and Tyrone (5 km. 2 ) respectively, but the study text does not indicate the proximity of the. systems to the stream. . Arnscheidt et al. (2007) observed that the TP concentration in all three catchments was elevated. compared to oligotrophic waters, with the highest concentrations occurring in the catchments with. the highest DWWTS density - Armagh and Monaghan. In addition, a strong correlation was observed. between the median TP concentration and the DWWTS to area ratio, particularly in the Armagh. catchment, where simple hydraulic pathways occur due to the narrow catchment shape. . Neither of these findings provides clear evidence that DWWTS were the cause of impaired surface. waters, and the authors conceded that agricultural point sources could also be an influential factor.. However, the TP evidence was supplemented by the use of markers of domestic wastewater in the. surface water environment. Source-specific faecal sterol signatures in the sediment at the catchment. outlets indicated that, at some point, human waste was likely to have entered these streams, with. sterol profiles indicating percentage human influences of 27% (Armagh), 7% (Monaghan) and 21%. (Tyrone) in the study catchments. In addition, correlations at varying levels of significance were. observed between P fractions and the human sterol signatures in each of the catchments, though the. catchment with the highest DWWTS density had the lowest percentage of human faecal sterol. signature. The evidence supplied in this study indicates a potential input of DWWTS to surface. waters, but does not prove it definitively, nor does it provide an estimate of P loading from DWWTS. to the catchment stream. . Jordan et al. (2007) provided further insights into the potential impacts of DWWTS in the same 5 km 2. Co. Tyrone catchment examined in Arnscheidt et al. (2007), observing that storm-driven processes. transported 92% (255.5 kg-P) of the observed 278.9 kg-TP load during seven months of high. frequency TP and flow monitoring, largely attributed to runoff from lands treated with animal slurry. (Jordan et al., 2007). However, when base flow conditions prevailed, in-stream TP levels remained. elevated at 0.25 - 0.5 mg-P L -1. . Sub-hourly P data revealed diurnal patterns in TP fluctuations during. extended periods of baseflow, potentially reflecting water usage patterns arising from chronic point. 16. sources. “Individual farmyards and/or septic tanks” (Jordan et al. 2007) were attributed as the cause. of elevated P at low flow (Figure 2.2), however, it should also be noted that other sources of P may. be the cause – such as in-stream processing of P, and diurnal cycling. TP and TDP levels have been. observed to increase in DWWTS effluent during spring and summer (Richards et al., 2016a), which. may further exacerbate the potential impacts from DWWTS during the prolonged low flow periods. which generally occur in summer. . Figure 2.2 Hourly TP concentrations observed by Jordan et al. (2007), illustrating median, upper and lower quartile,. maximum and minimum values throughout long-term low-flow periods. The data appear to indicate that point. sources of P cause a rapid increase in TP concentration at approximately 11:00 hr each day which is sustained until. the evening hours when concentrations appear to recover once more. (Figure is from Jordan et al. (2007), Creative. Commons non-commercial Sharealike 2.5 Licence). Concurrent, large spikes in conductivity and TP outside of rainfall events provided further evidence of. the influence of point source inputs to the stream. These discharge events would have been missed. under routine (e.g. weekly or monthly) water quality monitoring, which highlights the need for. intensive monitoring programmes with respect to understanding frequency and timing of unknown. point source discharges – information which can provide greater insight into the source. . Other studies have also shown degradation of water quality downstream of DWWTS compared to. upstream (e.g. Palmer-Feldgate et al. 2010), particularly at low flow (May et al. 2012). A similar. finding was made in a UK study (Withers et al. 2011) whereby statistically significant increases in. stream P, NH4 and NO2 concentrations occurred between upstream and downstream of a small. village (Loddington North) featuring 4 DWWTS (approximately 8 PE) and a visitor centre (Village East). receiving approximately 600 – 1,100 visitors per year (Figure 2.3). . 17. Figure 2.3 Observed changes in phosphorus concentration at monitoring stations upstream and downstream of two. surface water quality monitoring stations sites (Village East and Loddington North) potentially influenced by. DWWTS (from Withers et al. (2011), copyright reuse license obtained from Elsevier).. On average, phosphorus increased from 0.07 mg-P L -1. upstream to 0.21 mg-P L -1. downstream, with. the highest P concentrations (dominated by the DRP fraction) occurring during low flow periods. which, the authors argue, thus highlights the influence of point sources rather than diffuse sources. (Jarvie et al., 2006 and Jarvie et al., 2008) and the inability of the small headwater streams to. assimilate point source contaminant loading. The catchment in question was underlain by clay-rich,. poorly permeable soil, giving rise to the potential for development of preferential flow paths from. STAs to stream, direct inputs of untreated domestic effluent via pipelines, or overland/shallow. subsurface flow. MacIntosh et al. (2011) directly attributed increases in low flow P concentrations to. increased density of DWWTS in their study catchments. Richards et al. (2016) also observed increases. of in-stream SRP, TDP, TP and TN concentrations downstream of just a single DWWTS compared to. upstream, indicating that the density of systems in a given catchment may not be as important as the. proximity of just a small number of systems to a surface water feature. . McCarthy et al. (2012) monitored surface water quality in Co. Monaghan upstream and downstream. of the estimated region of interaction between five different DWWTS plumes and the associated. river channel for a number of DWWTS located in subsoils with T-values ranging from 53 to >90 (i.e.. unsuitable for the construction of conventional DWWTS). No significant differences in contaminant. concentration, including DRP and TP, were observed upstream and downstream of the. DWWTS/stream zones of interaction in this study, even at two sites where direct discharges to. surface water were observed, which suggests that the monitoring frequency was potentially too low.. Other studies have found that monitoring water quality at low flow and at a high sampling frequency. is critical for detecting the influence of DWWTS effluent in surface waters (Jordan et al., 2005(a) and. Jordan et al., 2007).. 18. The study by McCarthy et al. (2012) concluded that the SRP and TP are likely to be attenuated to. within background soil water concentrations, with the exception of sites where direct discharges. occur, but that DWWTS represent potential sources of contaminants to surface waters where poorly. permeable soils give rise to the occurrence of shallow preferential flow paths. Where surface drains. and small streams intersect DWWTS plumes, particularly those featuring preferential flow pathways. or direct discharge pipelines, there is potential for significant transfer of P to streams by DWWTS.. Jarvie et al. (2006) and Mallin and McIver (2012) postulated that rising water tables underlying STAs. potentially form an effective hydraulic pathway between the STA and the nearest water body, with. the transfer occurring along the transitional zone (TZ) (i.e. the interface between the base of the. subsoil layer and the upper bedrock zone) and/or the interflow pathway, depending upon the depth. of subsoil present (Archbold et al., 2010). The ditches which form the myriad network of artificial. drainage channels in poorly-drained catchments generally exhibit extremely low flow volume outside. of storm events. In cases where these drainage ditches receive direct discharges of DWWTS effluent,. the concentration of some constituents in the drainage network is likely to be similar to that. observed in DWWTS effluent, by virtue of the low assimilation capacity (Richards et al., 2016b). . The literature related to the impacts of DWWTS on surface water to date, generally indicates that a. potential impact exists. However, there is little quantifiable evidence of the specific impact of these. systems. For example, Mallin and McIver (2012) linked increased P concentrations in surface water. with increased use of local DWWTS during high tourism season, but the study did not include an. estimate of the amount of P likely to be discharged by these systems. Similarly, an in-depth. examination of the constituents of DWWTS effluent by Richards et al. (2016b) and their level of. detection in a nearby stream did not explicitly define the nature and scale of impact.. A useful measure of the impacts of DWWTS on surface water is the change in water quality that. occurs when one or more problematic systems are replaced or repaired. In general, the evidence. linking small-scale improvements in DWWTS infrastructure to improved water quality is not. definitive. In two of the catchments studied by Arnscheidt et al. (2011), the effects on water quality. of improvement works carried out on catchment DWWTS were inconclusive. In-stream DRP. concentrations reduced, but the overall annual catchment TP loading remained consistent with. previous years. However, Macintosh et al. (2011) observed an estimated reduction of 0.032 mg-TP L -1. when improvements were made to a small number of problematic DWWTS in the associated study. catchment. It is possible for just one faulty DWWTS to result in impaired water quality (Richards et al.. 2016b) and as such it is unknown whether the improvement in water quality observed by MacIntosh. et al. (2011) was due to improvement works on one or many DWWTS, or to concurrent efforts to. reduce agricultural P losses at the catchment scale. . 19. Not all of the literature indicates that P loading from DWWTS is a consistent problem across all. catchments, however. Gill et al. (2009) measured ortho-P levels in DWWTS effluent, at varying. depths beneath a range of STAs and across a variety of drainage characteristics and population. equivalents (P.E.), with results indicating that an average P reduction of 12% occurred in the primary. settlement tank due to the retention of solids, and that up to 90% P removal was observed at a. depth of 1 m at most of the study sites (100% removal measured at one site – see Table 2.1).. However, these data relate to relatively well-maintained DWWTS, likely to have been constructed in. line with the CoP (EPA, 2009), and the P removal efficiencies observed are unlikely to be. representative of DWWTS constructed on poorly-drained soil.. Table 2.1 Average ortho-P loading (g d-1) of influent and of soil water samples collected at various depths beneath six. DWWTS (adapted from Gill et al., 2009). Copyright reuse license obtained from publishers (Elsevier).. In secondary treated effluent from package treatment systems, s
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